What issues might there be because the Constitution did not spell out how a Vice-President would become President?

Two of these describe "federalism." (choose two answers) select one or more: a. the national government absolutely controls all state governments. b. power is shared by the national and state governments. c. state governments exercise absolute power over the national government. d. it is a compromise meant to eliminate the disadvantages of a "unitary" and a "confederate" system.